1. Field of the Invention
The present invention relates to glass joint bodies which are exposed to corrosive substances, and especially relates to glass joint bodies to be used for a connection between .beta. alumina and insulation ceramics in Alkali Metal Thermo-Electric Converters (AMTEC) or a secondary cell to be used in high temperatures such as a sodium-sulfur cell. The present invention also relates to a method of manufacturing the glass joint bodies mentioned above.
2. Related Art Statement
Usually, as for one example of glass joint bodies which are exposed in corrosive substances, there is known a sodium-sulfur cell or Alkali Metal Thermo-Electric Converter.
The sodium-sulfur cells are high temperature type secondary cells which operate at 300.degree. C..about.350.degree. C. and include metallic sodium as a cathode active material, sulfur and/or sodium polysulfide as an anode active material, a sodium ion conductive ceramic as a solid electrolyte, and a metal container. The structure of a typical sodium sulfur cell is shown in FIG. 1.
In FIG. 1, the numeral 1 is a beta alumina tube, 2 is a metal container functioning as an anode, 3 is a sulfur or sodium polysulfide, 4 is a metal container functioning as a cathode, 5 is sodium, 6 is an insulator such as .alpha.-alumina, 7 is a metal lid, 8 is a welded portion, and 9 is a conjunction glass for connecting the beta alumina tube 1 and the insulator 6. As for the beta alumina material forming the beta alumina tube 1, use is made of .beta."-alumina, .beta.-alumina, and a mixture of both, or the like.
Processes for manufacturing the above-described sodium-sulfur cells generally comprise the steps of: bonding the open end periphery of the beta alumina tube 1 with the ring insulator 6 made of .alpha.-alumina by means of glass or the like; bonding the ring insulator 6 supporting the beta alumina tube 1 with the metal containers 2 and 4 by a solid phase reaction or the like at a high temperature under pressure; supplying the sodium 5 and the sulfur or sodium polysulfide 3 into the metal containers 4 and 2 respectively; and hermetically closing the metal containers 4 and 2 with lids 7 and 8 by means of welding to provide a cell.
In the sodium-sulfur cells mentioned above, the conjunction glass 9 arranged between the beta alumina tube 1 and the insulator 6 made of .alpha.-alumina and the like is corroded by the sodium, and consequently a life of the sodium-sulfur cell is decreased. To eliminate the drawback mentioned above, a conjunction glass having good durability against sodium corrosion consisting of 1 wt % alkali earth metal oxides or less, SiO.sub.2 : 65.about.75 wt %, B.sub.2 O.sub.3 : 10.about.25 wt %, and the balance of Al.sub.2 O.sub.3 and alkali metal oxides is disclosed in Japanese Patent Laid-Open Publication No. 1-54672.
The conjunction glass having the composition mentioned above has good durability against sodium corrosion as compared with known silicate glass and boron silicate glass, but does not show a sufficient durability against sodium corrosion as yet. Therefore, the conjunction glass mentioned above is also corroded by the sodium and thus there is a drawback in that a life of the sodium-sulfur cell is also decreased.